# Netforce of spring, hookes law. help!

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1. Jan 23, 2015

### jim gerth

1. The problem statement, all variables and given/known data
An "extreme" pogo stick utilizies a spring whose uncompressed length is 46 cm and whose force constant is 1.4 x 10^4 N/m (14000). A 60 kg enthusiast is jumping on the pogo stick, compressing the spring to a length of only 5.0 cm at the bottom of her jump, Calculate; a) The net upward force on her at the moment the spring reaches its greatest compression and b) her upward acceleration in m/s^2 and gs at that moment.

Any thoughts?
Thanks.

2. Relevant equations

F=-kx F=ma w=mg
3. The attempt at a solution
Ok so i started with multiplying the force constant and difference of distance . 14000 x (0.46 - 0.05) = 5740 then i subtracted the weight which is (60)(9.8)= 588 which gave me an answer of 5152. The answer in the book says 5100. I know with having 2 sig figs you could say it was 5100 but i am not sure if i went about this right.. and also dont know where to go with b especially if i did not get a correct.??

thanks everyone.

2. Jan 23, 2015

### Shinaolord

Having 2 significant figures with your final result does NOT give 5100. It gives 5200.

$\vec{F}_{net} = \vec{F}_{spring} + \vec{F}_{person}$
Both of which you stated the formulas for.
If you round both of your results with respect to the amount of significant figures before subtracting, you get 5100 ( after significant figures are applied to this as well, beforehand it is 5110)
That's the only thing "wrong" I see.

Your analysis was correct, however. What are your thoughts on the second portion (the acceleration)?

Last edited: Jan 23, 2015
3. Jan 23, 2015

### jim gerth

Thanks for the reply and clarification, Yeah sorry , i meant to say 5200.. and i have no idea how to approach the acceleration , we have yet to learn that. ive been checking everywhere on how to approach it but i dont even know where to start lol -____- answers in the back is 85 m/s^2 and 8.7 gs. i wish i could say my answers were even close .

4. Jan 23, 2015

### jim gerth

lol i thought it was more than just that but nvm i got it . Thanks again